In a digitally controlled printing system, such as an inkjet printing system, a print media is directed through a series of components. The print media can be a cut sheet or a continuous web. A web or cut sheet transport system physically moves the print media through the printing system. As the print media moves through the printing system, liquid, for example, ink, is applied to the first side of the print media by one or more printheads through a process commonly referred to as jetting of the liquid. The jetting of liquid onto the print media introduces significant moisture content to the print media, particularly when the system is used to print multiple colors on the print media. Due to its moisture content, the print media expands and contracts in a non-isotropic manner often with significant hysteresis. The continual change of dimensional characteristics of the print media often adversely affects image quality. Although drying is used to remove moisture from the print media, drying too frequently, for example, after printing each color, also causes changes in the dimensional characteristics of the print media that often adversely affects image quality.
FIG. 1 illustrates a portion of the print media 112 as the print media passes over two rollers 68 and 70 that support the print media under each row of printheads in accordance with the prior art. The print media 112 is moving in a direction of media travel 60, also referred to as the intrack direction. Perpendicular to the intrack direction 60, and to the local normal to the surface of the print media, is the crosstrack direction 62. During the inkjet printing process, the print media can expand as the print media absorbs the water-based inks applied to it. When the direction of expansion is in the direction that is perpendicular to the direction of media travel 60, it is often referred to as expansion in the crosstrack direction 62. Typically, the wrap of the print media around a roller of the inkjet printing system produces sufficient friction between the print media and the roller that the print media is not free to slide in the crosstrack direction even though the print media is expanding in that direction. This can result in localized buckling of the print media away from the roller to create lengthwise ripples, also called flutes or wrinkles, in the print media. Flutes or ridges 64, 66 can be produced in the print media due to expansion of the print media in the crosstrack direction 62 because the print media cannot slip on the rollers 68, 70. Wrinkling of the print media during the printing process often leads to permanent creases forming in the print media that ultimately affect image quality.
Multiple printheads are typically located and aligned by a support structure to form a linehead, with the linehead located over the print media. In many such systems, the support structure of the linehead locates the printheads in two or more rows; the rows positioned parallel to each other and aligned in the crosstrack direction. To prevent the print media from fluttering, or vibrating up and down in the print zone, the print media is supported by a roller that is aligned with the print line of each row of printheads. It is not uncommon for the bottom face of the support structure to become wet, either due to condensation from the moist air produced by the printing process or due to mist drops created by the print drops striking the print media.
It has been found that under some printing conditions the flutes in the print media can be sufficiently tall that top of the flutes can contact the bottom face of the support structure. When this occurs, the moist ink on the flutes can be smeared by the contact. Additionally, the moisture on the bottom of the support structure can be transferred to the print media. The result is a degradation of the print quality.
Vacuum can be applied to the print media 112 to deflect the print media away from the linehead, thus reducing the formation of flutes or wrinkles in the print media. When the print media is deflected away from the linehead, there is less likelihood of smearing of the moist ink on the print media due to accidental contact with the linehead. One of the issues with applying vacuum to the print media is ensuring that the vacuum is applied uniformly across the print media in the cross track direction. If the vacuum manifold opening is larger than the width of the print media, there is leakage of air that can result in the print media not being deflected away from the linehead. If the width of the vacuum manifold is less than the width of the print media, the portion of the print media not under the influence of the vacuum force is not deflected away from the linehead, increasing the likelihood of smearing of moist ink on the these portions of print media due to accidental contact with the linehead. Furthermore, the portions of the web not under the influence of the vacuum can flutter, resulting in print quality degradation. There remains a need to better manage the vacuum, provided by the vacuum assembly, near the edges of the print media.